The present invention relates to an imaging system for imaging a subject and more particularly to an imaging system and an image defect correcting method for correcting an image defect of a radiographic image produced by a radiographic imaging system that uses a flat-panel radiation detector.
Radiographic image detectors that detect radiation (e.g., X-ray, alpha ray, beta ray, gamma ray, electron beam, and ultraviolet ray) that has penetrated a subject as an electric signal are conventionally used to produce diagnostic images in medicine or to conduct nondestructive tests in industry.
The radiographic image detectors are exemplified by a radiation detector, i.e., a so-called flat panel detector hereinafter referred to also as FPD, which converts radiation into an electric image signal, and an X-ray image display that converts a radiographic image into a visible image.
The type of radiographic image detector using an FPD includes, for example, a direct type that, in a manner, converts radiation directly into an electric signal by reading as an electric signal electron-hole pairs generated by a photoconductive film such as one formed of amorphous selenium in response to incident radiation.
One of the causes responsible for image degradation of radiographic images produced by a radiographic imaging system using such an FPD is defective pixels of the FPD.
All of the pixels or radiation detecting elements of the FPD do not necessarily produce an output signal having a proper intensity (density) in relation to the amount of incident radiation; there may be defective pixels that produce an output signal with a lower or higher intensity than is appropriate in relation to the radiation applied, which is attributable, for example, to defects that occur in manufacture.
As will be expected, defective pixels fail to produce a proper radiographic signal. Images corresponding to such defective pixels (image defect) may cause serious problems such as false diagnosis.
In addition, defective pixels of the FPD tend to increase with the number of times radiographic images are taken.
It is therefore a normal practice with a radiographic imaging system using an FPD to have located defective pixels of the FPD at a given timing and, when actually taking a radiographic image, correct the image defect by using data of neighboring pixels (the corresponding image data) according to the defective pixel detection results to ensure that a radiographic image of which the image defect has been corrected is displayed or printed out by way of reproduction for diagnosis or other purposes.
To detect defective pixels of an FPD, one may, for example, employ a method using a radiographic image to detect an image defect in the radiographic image and detect the defective pixels of the FPD corresponding to that image defect.
Such a method for detecting defective pixels of an FPD, however, may fail to detect an image defect in a radiographic image (radiographic image data) with an optimum size because of noises such as random noise.
When an image defect in a radiographic image cannot be detected with an optimum size, appropriate correction of the image defect in the radiographic image is impossible.
In addition, when detecting defective pixels of the FPD by the above method, there are cases where the area of the defective pixels of the FPD is found to have grown larger at the time when a radiographic image that needs image defect correction, i.e., a radiographic image of a subject, is taken as compared with the time when a radiographic image used for detecting an image defect was taken.
In such cases, even when the area where an image defect will occur is previously detected with an appropriate size, the size of the image defect in the radiographic image representing a subject may be larger than that of the image defect in the radiographic image used for image defect detection so that the area where image defect correction is effected may not be large enough to cover the area of the actual image defect, thus failing to provide an appropriate image defect correction.